A converter for converting three-phase alternating current (AC) to direct current (DC) is used for, for example, a motor driving inverter, a battery charger/discharger or a refrigerating apparatus (such as air conditioner or refrigerator). When the converter uses a three-phase diode rectifier, many power source current higher harmonics are generated, having an influence upon an electric power system to now give rise to a social problem.
In recent years, higher harmonics regulations, including those by IEC (International Electrical Standards Conference): LEC61000-3-2 (phase current<16 A and IEC61000-3-12 (16 A<phase current<75 A), have been established in Europe, China and Japan. In future, countermeasures against power source higher harmonics in those apparatus are expected to be necessary.
On the other hand, by using a three-phase PWM converter constituted by 6 semiconductor power devices, reduction of higher harmonics in input current and stabilization of output DC voltage can be controlled but many semiconductor power devices and a complicated control means are needed, thus raising a problem that costs of apparatus increase to a great extent.
Especially, in civil and industrial apparatus such as air conditioners, general-purpose inverters and electric motorcar chargers which make much account of costs of products, inexpensive countermeasures against higher harmonics have been desired.
Conventionally, as inexpensive countermeasures against higher harmonics in the three-phase converter, a method has been proposed, as described in “Patent Literature 1” and “Patent Literature 2” for instance, according to which an AC reactor and three both-way energizing type switches are provided on the input side of a three-phase diode rectifier and the both-way energizing type switches are turned on nearby a zero-cross alone of power source voltage of each phase so as to improve the input current.